Can you post the full output of the error, including also the commands you are running to attempt to build the images ?
Note that rhel is a bit special compared to the rest of the distros as it requires sensitive information from the subscription. If you notice, in the makefile there is no make rhel despite having the packer files to build them.
You have to define a Makefile.local with this information and the entrypoint for rhel8 and 9. Something like this should help
DISTROS += rhel8 rhel9
PKR_VAR_rhel_user := your_rhel_user
PKR_VAR_rhel_password := your_rhel_pass
PKR_VAR_rhel := { 8 = { iso_url = "http://path/to/rhel8.qcow2", \
iso_checksum = "the hash of this image" }, \
9 = { iso_url = "http://path/to/rhel9.qcow2", \
iso_checksum = "the hash of this image" } }
In our tooling, the iso_url variables actually point to prebuilt qcow2 images that we modify, perhaps we should change the name of the variables to make them more intuitive for users. Is similar to how alt linux is built.